1. Field of the Invention
The present invention generally relates to electromechanical transducers and, in particular, to transducers that directly convert an electrical signal into rotational mechanical action.
2. Background Art
In one of applicant's prior patents, U.S. Pat. No. 4,928,030, he describes electromechanical actuators that combine lifters and tangenters, which respectively translate a responsive surface perpendicular to, and in the plane of, the responsive surface in response to separate electrical signals. The mechanical strokes of the described transducers are linear. Compound transducers are taught that combine tangenters and lifters in various ways to provide one, two, and three linear motions. Devices such as linear, rotary, and combined linear and rotary motors are described. Also described are the methods of positioning an object with single and compound linear actuators using traction and release with substantially differing velocities, and using walking without sliding between each actuator's traction surface (crown) and the walked surface of the positioned object. The rotary motors have no gross sliding. They use a rolling line contact in which contact pressure is relatively high, and at which microrubbing is unavoidable. Other taught motors allow sliding by a twisting action between the crown and the traction surface of the positioned object. Gross twisting sliding and rolling microrubbing reduce electromechanical efficiency by dissipating a portion of the available electrical power as frictional heat, and shorten the life of the motors by wearing the traction surfaces.
Applicant's copending application, Ser. No. 07/488,548 filed Mar. 15, 1990 teaches an electrical drive means that elicits a nonsinusoidal walking mechanical waveform from the responsive surface of an electrically segmented transducer, taught benefits being increased mechanical efficiency due to reduced gross sliding during walking positioning of an object, and increased electrical efficiency due to electrical but not necessarily electromechanical resonance. The teachings are generally applicable to the preponderance of electrically segmented electrodeformable transducers, and particularly to walking transducers. The teachings are directed primarily toward transducers that produce linear strokes, but are considered to be encompassed by the scope of the present invention because the advantages of these teachings apply equally to twisting as well as to linear transducers. By way of example, a transducer walks a curved path with greatest efficiency when its "ankle" twists by an amount predicated on the rotation subtended by a step of the curving path. An improper amount of twist results in some combination of ankle strain and sole wear. Highly efficient walking is most effectively achieved by both linear and, by inference, by rotary nonsinusoidal mechanical stroke waveforms.
In Technical Reference EMDUSM-8703 "Ultrasonic Motor" Panosonic Industrial Co. describes an ultrasonic traveling wave motor that is representative of a diverse class of devices, all known embodiments of which use electromechanical resonance to improve electrical efficiency. However, the vibration of mechanical resonance is characteristically sinusoidal, resulting in an elliptical output stroke. Contact of the rotor surface with wave crests of an elliptically oscillating vibrator entails contact sliding to the extent that more than half of the applied electrical power is dissipated as frictional heat. The rubbing engenders a relatively short contact surface life due to wear. The preponderance of ultrasonic traveling wave motors hold the stationary resonant wave plate against the rotating plane surface of a disk rotor, causing twist rubbing that adds wear and frictional heat generation to that resulting from the elliptical contact motion.
Japanese patent 63-274,894 to Uozumi describes two-axis electrodeformable transducers that use a walking action to position a tunneling electron microscope stage in three directions, but primarily in two directions in the plane of the sample. The reference also describes a motor having 12 similar transducers that walk the broad planar surface of a rotated disk in alternating groups. Each described transducer provides two linear motions, one normal to the plane of the disk, and another tangential to the disk axis. However, during each traction portion of a walking step, the surface of the disk rotates while the transducer executes linear translations, thus producing a first rubbing that is radial due to a minute change in radius that is the difference between a tangential stroke and a truly annular stroke, and a second rubbing caused by differential twist between the linear transducer and the rotating disk surface. Rubbing reduces electromechanical efficiency through frictional dissipation, and reduces life due to wear.
Japanese patent 60-20,775 Ogiso, describes a piezoelectric motor having six, two-axis transducers that walk the cylindrical surface of a rotor in alternating groups. The traction member (10) of each walking transducer has a plane traction surface that is forcefully positioned in a first radial direction relative to the rotor axis, and in a second direction tangential to the cylindrical surface of the rotor. Activation of the motor is described in a context of electromechanical transducer resonance that positions any point of the plane of the traction surface along an elliptical path, but does not rotate the plane. The rotor is rolled between opposing traction members, the rolling occurring at line contacts between the planes of the traction members and the rotor's cylindrical surface. Relying on traction, the generable tangential force is less than or equal to the product of the radial force and the traction transducers can be used in actuators to provide angular forces in coefficient of nonsliding friction. Extraction of significant torque from a motor of this type entails relatively large radial forces which, in the area immediately surrounding the line contact, causes compressive and shear stresses that easily approach the endurance limits of most materials. An ideal motor of cylinder rotor type may use a traction member having a contact surface that is a segment of a cylinder having the same curvature as the rotor, thus distributing the pressure due to radial force over the entire area of contact. Clearly, the two cylindrical surfaces must have coincident axes in order to assure a proper fit, and transducers that combine linear motions cannot provide coincidence at any position other than at the center of each walking stride.
Variants of walking motors and actuating devices having toothed traction surfaces, when only one of the toothed surfaces is curved, produce an actuation force that is limited because relatively few of the teeth are engaged at any particular phase of activation. Fewer teeth bear the operating load, thereby incurring a higher tooth pressure and stress that would otherwise prevail when all possible teeth were engaged and equally sharing the load.
U.S. Pat. No. 4,868,447 Lee et al, describes transducers made of polyvinylidene fluoride (PVDF) piezoelectric material laminated to other similar layers, or laminated to another body, in a manner that allows transduction by twisting and by bending. The polymer film taught (1:27): ". . . only generates normal stress and strain . . . ". Further, Lee et al teach (7:37): "However, due to the thin film shape of PVDF, the [shear] constants d.sub.15 and d.sub.24 remain unknown. Equation (7) reveals that by applying an electric field along the z-axis, only the normal strain will be induced. This explains why all previous applications of PVDF bimorph are primarily in the bending mode." Lee et al teaches a new composite piezoelectric theory (7:43 et seq) wherein (15:63) ". . . it is desired to generate shear forces by normal stresses . . . " when a composite transducer is made of individual PVDF layers, each layer deforming solely in the extension mode, and in which the composite, as a whole, detects or actuates compound bending and twisting by the interaction of the extension of a first extension layer with the extension of a second extension layer angularly disposed to the extension of said first extension layer. Lee et al teach that twist is the sum of two bending deformations due to extensions of at least two angularly disposed broad surfaces, all of said broad surface being deformed, that is, not having the quiescent shapes of the broad surfaces preserved, during twist.
Lee et al describe a pure torsion embodiment of the laminated transducer (29:4) that requires at least two piezoelectric layers and a sandwiched inert shim layer. Lee et al teach that the twist occurs about a normal of the plane containing the narrow end edge of the laminate (6:2 and FIG. 1), this normal being the axis of least stiffness. Lee et al allude to stresses internal to the taught laminate (12:21): ". . . each lamina that has its own displacement and can be related to the displacement of the other laminae by inter-lamina boundary conditions . . . " indicates that strains achieved by laminates will always be of lesser magnitude than free-lamina strains because of the stressed interlaminar boundary conditions that prevail during both twist and bending relegate a portion of the otherwise available free-lamina transduction to the generation self-canceling internal stresses. The transduction losses due to interlaminar stress is sufficient to have fostered a literature of corrective measures, a representative being U.S. Pat. No. 4,649,313 Ogawa et al, which describes a buffer layer at or near the neutral fiber of a piezoelectric bending transducer, the buffer layer acting to ameliorate internal (shear, due to bending) stress to a prescribed limit, and thereby allow the transducer to dedicate a greater share of the transduction to desired displacement than would otherwise occur because of generally self-canceling interlaminar stresses.
The teachings of Lee et al include polarization of the piezoelectric material by applying an electric field across the thickness of the film, then activating the film by the application of an electric potential that induces an electric field in the same direction as prevailed during polarization (poling). It will be readily apparent to those versed in the particular art that the application of an activating potential to the film that results in an induced electric field comparable in strength to the original poling field, but in the opposite direction, may cause the original strength of poling to be reduced, cancelled, and in extreme cases, the poling may be partially or wholly reversed, resulting in a transducer that responds in the opposite sense as originally intended. Therefore, the transducers of Lee et al are limited, under these conditions, to essentially monopolar electric drive.
Claimed by Lee et al (claim 11) is polarization" . . . wherein the polarization profile of each lamina is varied in magnitude and direction to vary the response characteristics of the lamina." The corresponding teachings of Lee et al define profile as separate areas of a lamina, any one area having a single uniform polarization in one of two possible directions, either perpendicular and into a top side, or perpendicular to and out of a top side. The corresponding teaching defines "response" in this context as the dynamic response to a particular mode of oscillation of the lamina and attached structure, for example, to sense "mode 2" (37:11 et seq). The corresponding teachings define varied in magnitude as full positive, full negative, or zero, in accordance with the particular area of the profile. Smoothly varying the magnitude from one value to another value in a particular area is not taught, and would serve no identified purpose in the context of a profile designed to be responsive to a particular vibrational mode. Directions of polarization other than normal positive and normal negative relative to a top surface are not taught. Lee et al teach the polarization of piezoelectric polymer films using mechanical means in combination with electrical means. Means of smoothly varying magnitude and direction of polarization, other than in the afore referenced magnitudes and directions, are not taught by Lee et al, and further, are prohibited by the mechanical-electrical nature of the piezoelectric polymer polarization process. Therefore PVDF, the most common and representative piezoelectric polymer, is excluded from the shear class of electrodeformable transducer materials.
The teachings of Lee et al are clearly distinguishable from those of the present invention by the following differences:
In Lee et al, the transducer laminate is made of extension mode material, thickness mode material, or at least two extension mode materials mutually angularly disposed (whereas in the present invention, a body portion is made of shear electrodeformable material);
In Lee et al, the broad surfaces are deformed during deformation (whereas in the present invention, a layer, including the layer with the thin film shape, produces a twist of one broad surface relative to the opposite broad surface while essentially preserving the shape of both broad surfaces);
In Lee et al, a portion of the transduction is relegated to self-canceling internal stresses that reduce the transduction that would otherwise be available if said internal stresses did not obtain (whereas in the present invention, because the shapes of broad surfaces of a single layer remain undeformed by twist, the layer produces the same fullness of transduction in the free layer as is produced in a layer having one or both broad surfaces attached to other layers, or attached to other rigid members);
In Lee et al, twist is achieved by combining two or more extensions having relative angular disposition (whereas in the present invention, twist is produced by shear deformation of a single layer without recourse to ancillary structures);
In Lee et al, electric drive is limited to essentially monopolar drive (whereas in the present invention the use of shear electrodeformable material permits bipolar electric drive and therefore provides at least twice the total transduction with equivalent comparable magnitude of strain and stress);
In Lee et al, the polarization of a lamina has a profile, that is, each areal portion of a lamina may have a separate but uniform direction and magnitude of polarization (whereas an embodiment of the present invention smoothly varies the polarization over the entire area of a lamina because a profile consisting of separate uniformly polarized areas serves no purpose defined by the present invention);
In Lee et al, the polarization is varied in direction, said direction being perpendicular into and perpendicular out of a top surface (whereas in the present invention the direction of polarization is varied smoothly from one direction at one portion of a layer to another direction at another portion of the layer);
In Lee et al, the polarization is varied in magnitude, said magnitude being full positive, full negative, and zero relative to a top surface (whereas the present invention smoothly varies the magnitude of polarization from a first value at a first location of the layer to a second value at a second location of the layer); and
In Lee et al, twist occurs about a normal of the plane containing the narrow end (or side) edge of the laminate, this normal being the axis of least torsional stiffness (whereas twist of the present invention occurs about the normal to a broad surface, the stiffest twisting axis).